A practical endodontic filling composition has numerous requirements. It must not shrink excessively on setting, thus unsealing the root canal. It must set within a time which reasonably permits completion of endodontic procedure without undue patient discomfort. It must be biologically compatible with tooth structure and non-toxic. It must be inert to moisture and to the acid-base conditions found in the mouth. Is is preferably X-ray opaque. It should also be of low viscosity to facilitate ready insertion into and complete filling of the canal itself prior to setting. It should be easy to remove after setting if necessary or desirable. To permit insertion and retention in root canals of upper teeth before curing, it must be thixotropic. It should not discolor or stain tooth structure on prolonged residence in the mouth.
Systems which can be injected by means of a syringe have also been used as endodontic filling materials. One such system uses zinc oxide and eugenol. The two materials are mixed together to form a paste. This can be placed into a pressure syringe and injected into the root canal. The zinc oxide-eugenol mixtures polymerize when initiated by water. This material is a poor choice for several reasons. Adhesion to the walls of the canal is very low. It is very soluble and has little physical strength. It does not penetrate secondary canals.
A mixture of hydroxyethyl methacryate, fillers and catalysts has also been used as a syringeable endodontic filler since the monomer cures to a rigid plastic, which, after a few days, absorbs enough water to become a soft hydrogel. However, hydroxyethyl methacrylate is a serious skin irritant. The residual catalysts are also believed to be irritating. This material relies on the absorption of water to swell up and seal the canal so the seal is often incomplete.
Similarly, compositions based on epoxy resins have been employed as injectible endodontic filling materials. One such material, known as AH26, consists of an epoxy resin (diglycidyl ether of bisphenol A), fillers and hexamethylene tetramine. Although epoxy resin are generally considered to cure hard and rigid, this system cures slowly to a weak material. The epoxy resin is a bad skin irritant and sensitizer. The formaldehyde released in the presence of water by the breakdown of the hexamethylene tetramine is both an antiseptic and an irritant. Because of these drawbacks the material has not found widespread use.
Two part room temperature setting silicone compositions comprising a base material and a catalyst and the use thereof as dental impression material is described, for example, in U.S. Pat. Nos. 4,007,123 and 3,897,376. The base material of U.S. Pat. No. 4,007,123 comprises a hydroxy dimethyl siloxane, an organo-silicone cross-linker, and a unique filler, the catalyst component of which is a metal salt of a carboxylic acid, e.g., tin linoleate, stearate, oleate, acetate, or butyrate.
Two-component, room temperature setting organopolysiloxane compositions comprising (i) a hydroxy terminated diorganopolysilioxane mixed with a filler and (ii) a catalyst mixture of a tin carboxylate and an alkyl silicate or partially hydrolyzed alkyl silicate are described in U.S. Pat. No. 2,843,555. According to U.S. Pat. No. 3,897,396, it has been suggested that such two part compositions be placed in a syringe or other device and inserted from the device into the ear so that the composition can cure in situ to form an ear plug for the attenuation of sound.
Silicone compounds said to be useful as dental molding compounds and for filling tooth roots are described in British Pat. No. 841,825. U.S. Pat. No. 3,082,526 describes a single component, water or saliva cured silicone rubber system for filling root canals.
Silicone elastomers have been proposed for use as endodontic filling and sealing compositions. Such endodontic products so proposed were not formulated to enable application as a flowable liquid by a disposable syringe, e.g., a hypodermic syringe. Nor do prior writings concerning such proposals affirmatively show the achievement of a non-toxic root canal filling material inert to surrounding tissues and to mouth conditions, though it is to be recognized that such has necessarily been the desideratum.
For example, Nitzsche U.S. Pat. No. 3,127,363 describes a polysiloxane elastomer which cures at room temperature. The patent states that certain of the disclosed compositions are "especially suitable as root fillings for decayed teeth . . . because within a few minutes after having been pressed into the root channel they set to form a solid though still elastic mass . . . " (Col. 8, lines 33-40). Hydroxy terminated polysiloxanes are utilized as starting materials for polymerization with various metal salt catalysts incuding dibutyltin dicarboxylates. The Nitzsche patent discloses the use of cross-linking agents, but prefers such agents to be siloxanes present in relatively low amounts based on the weight of hydroxy terminated polysiloxane utilized. The patent discloses that "in certain circumstances" polyalkyl silicates may be used as cross-linkers (Col. 3, line 67-Col. 4, line 37), but lists numerous disadvantages connected with their use. In particular, greater shrinkage upon reaction of the hydroxy terminated polysiloxanes with cross-linkers is said to inhere if "polyethyl silicate" is the cross-linker than if RSi(OX).sub.2 or RSi(OX).sub.3 is so employed (Col. 4, lines 14-20). No particular formulation is exemplified as useful for root canal fillings. No reference is made to non-cross-linked compositions of a viscosity requisite to permit injection by syringe into the root canal. Some catalyst compositions of the type described in Nitzsche and the polymer obtained therefrom have been found to be biologically unacceptable.
Lewis U.S. Pat. No. 3,186,963 describes gelling a dimethyl polysiloxane at room temperature with a "vulcanizing agent" catalyst system which is a reaction product of a tin salt of carboxylic acid, e.g., dibutyl tin dilaurate and a silicate, e.g., ethyl orthosilicate. The reaction of the tin salt and the alkyl silicate is performed under ambient atmospheric conditions at 80.degree. to 200.degree. C. for at least 15 minutes, or under what the examples call "reflux" conditions using the same time and temperature criteria. The examples show that when an alkylpolysilicate is substituted for ethylorthosilicate, the catalyst similarly prepared can be "diluted" with excess alkyl polysilicate, but Example 2 indicates that the gel time is lengthened when such "diluent" is present relative to an equivalent mix containing no diluent. No information is given with respect to the viscosity of the catalyst, diluted or undiluted, or of the dimethyl polysiloxane starting material. No information is given about the characteristics of the gelled composition. Nor is reference made to the use of the composition as a root canal filling and sealing material.
Smith U.S. Pat. No. 3,957,704 describes unique cross-linking agents for two component room temperature vulcanizable silicone rubber compositions. These cross-linking agents are said to solve certain problems which attend the compositions of Lewis U.S. Pat. No. 3,186,963; namely variation in the cure rate and instability of the activity of the catalyst component upon storage. Dilution of the Lewis reaction product catalyst with a non-reactive dimethylpolysiloxane chain stopped with triorganosiloxy units is said to degrade the catalyst component seriously in that the shelf life was less than one to two months.